Traffic congestion is a significant problem in the United States as well as in many countries throughout the world, costing many billions of dollars annually. Although some people have and will in the future alter their commuting preferences in view of the state of traffic congestion problems, there is limited willingness of many people to do so. While “telecommuting” may reduce some traffic, workers and freight inevitably will need to be transported from one location to another. The transportation system should be improved to meet the needs of today's society.
Ground transportation systems typically comprise three major entities: links, nodes and vehicles. Links are the corridors, which may comprise for example, highways or railroads, mostly on the ground but occasionally in tunnels or elevated. Nodes may comprise for example, connections between two roadways or tracks, between a track and a side-track leading to a station or stop-off, or access to open roads and streets. Vehicles are automobiles, buses, trucks and freight cars, providing both person and freight transportation.
A single lane of traffic can carry 2000 cars or more an hour as the peak builds up, usually at commuting time. At this throughput level, any disturbance causes a severe disruption in traffic flow. Such disturbances may include traffic incidents (even in the opposite lanes), a driver slowing down, something taking away the drivers' attention and, in any case, an increase in traffic volume feeding in from other roads. The shortest achievable average headway (the time interval between two cars) in normal traffic, corresponding to a flow of 2000 vehicles per hour is 1.8 seconds. The headway does not imply any particular speed. But two extremes of speed should be noted.
First, at 200 mph, race drivers routinely maintain a separation of one car length or less, equivalent to a headway of 1/15 sec, some 25 times shorter than on the highway. The minimum headway on the road is dictated by what might be called the “average human reaction time” of two thousand people who drive often under normal road conditions. The reduced headway for race drivers is inconceivable for today's highway configuration, but is entirely conceivable on a special track utilizing automatic controls.
Second, at 10 mph or 15 ft/sec, a car 15 feet long would barely have time to get by a particular point on the road, requiring 1 second headway; a 40 foot coach would require a minimum speed of 27 mph (40 ft/sec); a 60 ft tractor trailer combination, 41 mph (60 ft/sec). At the lower speeds, traffic capacity is necessarily lower because the headway is necessarily longer to allow time for the vehicle itself to pass by. This is the speed barrier. Above this speed, capacity can increase, of course, but the controlling factor is then the headway.
In considering the implications of this data, assume that, with automated vehicle control, headway is halved to 0.9 second, allowing 4000 cars per hour (and from now on we shall refer to any vehicles in this context as “cars”, whether automobiles, coaches, trucks or freight cars of any capacity). If all cars are freight cars carrying a 20 ton load, one track has a freight capacity of 80,000 tons per hour; if all cars carry 50 passengers, the track capacity is 200,000 passengers per hour; or a mix of the two.
Congestion is the result of passenger cars carrying on average 1.6 persons per car. At today's maximum per-lane capacity of 2000 vehicles per hour, this amounts to just over 3,000 passengers per hour, in contrast with possibly 200,000 as stated above or even more. Therefore, one response to traffic congestion is use of mass transport; however, mass transport today comprises the railroad systems. Railroads have specific drawbacks, namely, they do not typically go where individual passengers or freight businesses want to go. Rather, the railroad may generally go from one area to another area, but additional transport is needed to get to and from the railroad station. Such additional transportation can be expensive and time-consuming; this is especially the case for freight businesses. As a result, the railroads have lost freight business to trucking, and passenger traffic to automobiles.
One common response to traffic congestion is simply to build more roads. However, adding one lane carrying 3000 passengers maximum or perhaps 1500 per hour on average for 24 hours, yields approximately 1.3×107 passenger miles a year. This is less than one thousandth of 1 percent of the total of 1.9×1012 passenger-miles a year totaled for the United States. As a result, adding 1000 lane segments in 1000 critical areas would yield approximately a 1% increase in the total, a marginal improvement at best. Furthermore, obtaining right-of-ways to build new roads or even widen existing roads is a difficult and sometimes, almost an impossible task.
Therefore, what is desired is a system that reduces traffic congestion while still accommodating the individual needs of individuals.
It is further desired to provide a system that increases the number of passengers and/or amount of freight that can be transported at any particular time.
It is still further desired to provide a system that provides a flexible approach to mass-transit by increasing the flow of cars moving along a track while still providing for transport of persons and/or freight from and to individual destinations.